Don't buy the Facebook app. It's a really poorly made product and not worth the time or the money.
As far as the Facebook options in your admincp, that is for Facebook Connect, which allows your users to login or register for your forum using their Facebook logins. Even this is partially broken with no resolution in sight. It doesn't work for users who use Internet Explorer and vBulletin says that its Facebook's fault and have no fix for it.
Facebook Connect is free, whereas the vbulletin Facebook app is a paid product (that you should avoid).
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